Prior art eddy current test apparatus has been utilized for inspecting metallic workpieces for flaws and irregularities. Eddy current testers employ a current carrying coil for inducing eddy currents in a workpiece surface in close proximity to the current carrying coil. To produce these eddy currents, an alternating current signal energizes the test coil causing electromagnetic radiation to impinge on the workpiece surface.
The response of the workpiece to this electromagnetic radiation varies with the structure of the workpiece. Flaws and irregularities in the workpiece modify the eddy currents and these modified eddy currents are monitored to obtain an indication of flaw and/or irregularity locations. Disruptions due to flaws or irregularities in the workpiece can be monitored with a test coil closely positioned in relation to the workpiece surface. In some eddy current applications, the test coil is separate from the energization coil but in certain instances, the test and energization coil are one in the same.
One use of eddy current test apparatus is to detect flaws in a workpiece as that workpiece is produced. In the manufacture of steel bar product, for example, it is advantageous to isolate seams, gaps or other irregularities of a certain severity before large amounts of scrap bar product are produced. This testing must therefore be performed while the bar is still quite hot just after it has been rolled so that corrective measures can be taken to avoid those flaws.
Pending U.S. application Ser. No. 625,029 of Harris et al, which was filed June 27, 1984 and is entitled "Eddy Current Flaw Detector Having Rotatable Field Defining Sleeve", discloses a recent development in eddy current testing. The disclosure of this pending application is incorporated herein by reference. Apparatus disclosed in this application utilizes a rotatable sleeve having apertures which periodically disrupt an electromagnetic field created by an energization coil. Monitoring circuitry coupled to the energization coil analyzes the effect the field disruption has on eddy currents in the workpiece surface. This disruption enhances changes in signal output due to a flaw or the like in the workpiece surface.
Flaw induced variations in energization coil output can be displayed on an oscilloscope. The existence of a flaw in the workpiece surface is exhibited by a spike or other variation in the oscilloscope sweep. Analyzing this type of signal requires skill on the part of the operator of the test unit. The height of the spike is referenced with respect to background signals which vary with noise and irregularities in the shape of the workpiece. The location of the spike along the oscilloscope waveform is not easily interpreted to determine the circumferential location of the flaw on the workpiece surface.
While the Harris et al development represents a significant improvement in eddy current testing for flaws, complete anaylsis of the workpiece condition still requires visual inspection of the workpiece and skill on the part of the operator using the Harris et al apparatus in interpreting the flaw information displayed with that apparatus.
In addition to testing for flaws of a certain depth it is important to recognize when the product is misshaped. An out-of-round steel bar may be scrap even though the product is free of flaws or seams in its outer surface. The specific shape of the misformed workpiece can give an indication of how the problem occurred and what steps can be taken to rectify the problem. The Harris et al apparatus does not address the problem of identifying an irregular or mis-shaped workpiece.